Generally, when many hundreds of points have to be joined by brazing as in the case of a heat exchanger, brazing sheets are used for component pieces such as plates or fins, the assembly being heated in a furnace after fitting all parts together and the brazing of plural points being accomplished in one heating step.
An example of the use of brazing sheets to assemble aluminum materials is in the construction of a heat exchanger body, where it was common to employ preplaced brazing using a preplaced brazing piece such as a pipe or the like. Alternatively, instead of a pipe, a powdered brazing paste was used as an auxiliary material in the brazing of the heat exchanger body. In both cases, the aluminum material to be brazed was an aluminum-silicon alloy.
In one method of brazing aluminum parts, the aluminum oxide film on the surfaces of the parts to be joined was removed by a flux. In recent years, instead of the chlorine type flux material which was previously used, a non-corrosive, non-water-soluble aluminum fluoride type flux has been developed. This fluoride type flux is used as a flux for the above furnace brazing step. As it is almost non-corrosive, non-water-soluble and non-hygroscopic when compared with the previous chlorine type flux, the fluoride type flux was dispersed in a liquid together with an aluminum alloy brazing powder immediately prior to use. This flux suspension was then coated onto the parts to be joined by spraying or dipping, and after drying the parts, they were assembled.
According to the prior art, the medium used to disperse this flux was generally water, however as the viscosity of water is extremely low, the adhesion of the flux to the parts to be joined was poor. If adhesion is poor, the flux easily falls off after coating when the coated surfaces are subject to friction and vibration. This adversely affects the working environment due to scatter of flux and decline of brazing properties, and leads to the build-up of foreign matter in the interior circuitry of the heat exchanger. Also, as a large amount of flux drips off during the coating process, excess flux had to be applied to cover the loss. This improves brazing properties, but leads to increase of flux residues on the joining surfaces after brazing by heating and again results in poorer film adhesion during the coating process. If these large amounts of residues retain moisture, anti-corrosion properties are impaired and an unusual odor is produced. Moreover, residues in the internal circuitry of the heat exchanger increased resistance to the flow of air or coolant and caused heat exchanging performance to fall.
Various flux compositions were therefore proposed to increase the viscosity of the flux suspension and improve its adhesion. In one such technique, the dispersant is an organic solvent, and a resin (e.g. carboxymethylcellulose, rosin, vinyl acetate) is added to increase the viscosity of the vehicle ("vehicle" being a general term for the dispersant of the flux or aluminum alloy powder, i.e. the resin and organic solvent). For example, JP 07-185796A suggests polyvinyl alcohol as an example of such a resin.
However, there was a problem with this flux composition in that when the temperature was increased up to the brazing temperature (approx. 600.degree. C.), the external appearance of the product was marred and defects in joins were caused by pyrolysis gases, voids in fillets due to carbonization and black surface residues. Also pyrolysis products also adhered to the aluminum brazing furnace.
Another technique is disclosed for example in JP 08-187594A, wherein a viscous vehicle, comprising a polyalkylene oxide resin dissolved in an organic solvent, is used as a dispersant. Suitable viscosity is obtained by dissolving the aforesaid polyalkylene oxide in the solvent, pyrolysis properties are satisfactory, and carbonization residues are not produced after brazing. This flux paste is applied to the parts to be joined, and after heating to vaporize or decompose the components of the vehicle, brazing is performed by the usual method.
However the organic solvent had an unusual odor, and the foul odor produced on heating gave workers an unpleasant feeling. Some organic solvents used in the flux paste such as 1,4-dioxane had a high flammability, were toxic to man, and affected workers' hygiene as a whole.
Moreover solvent remained in joins as undecomposed residue, causing deterioration of brazing properties.
When a viscous vehicle was used as dispersant, adhesion of flux was improved compared to the case when water was used, but some flux fell off from joins due to friction or vibration during work performed after coating and drying up to the brazing step. Hence, the problems due to peeling flux were still not completely resolved.